Dispersion is a known limitation in optical networks that causes a broadening of input pulses traveling along the length of the fiber. One type of dispersion relevant to the present invention is chromatic dispersion (also referred to as “material dispersion” or “intramodal dispersion”), caused by a differential delay of various wavelengths of light in a waveguide material. Dispersion has a limiting effect on the ability to transmit high data rates. When modulated onto an optical carrier, an optical spectrum is broadened in linear proportion to the bit rate. The interaction of the broadened optical spectrum with wavelength-dependent group velocity (i.e., dispersion) in the fiber introduces signal distortions. The amount of tolerable distortion is inversely proportional to the square of the bit rate. Thus, the combination of increasing spectral broadening and decreasing distortion tolerance makes the overall propagation penalty proportional to the square of bit rate. This results, for example, in a 10 Gbps signal being 16 times less tolerant to dispersion than 2.5 Gbps signal, while having only four times the bit rate. Dispersion accumulates linearly with propagation distance in the fiber and typical propagation distances in standard single-mode fiber (e.g., SMF-28 or equivalent) are about 1000 km at 2.5 Gbps, 60 km at 10 Gbps, and only about 4 km at 40 Gbps. Clearly, some form of dispersion compensation is required to obtain meaningful propagation distances at bit rates of 10 Gbps and above.
Dispersion compensating fiber (DCF) (also referred to as a Dispersion Compensation Module or DCM) has found widespread practical acceptance and deployment due to numerous advantages. Such advantages include relatively low loss and cost and the ability to simultaneously compensate channels across multiple wavelengths without requiring spatial separation. Further, DCF has the ability to compensate for the unavoidable variation in the dispersion as a function of wavelength (second-order dispersion or dispersion slope) that exists in many current transport fibers. To compensate for positive dispersion in a transmission fiber, conventional systems use lengths of DCF that have a negative dispersion coefficient. The length of DCF is selected so that the negative dispersion produced by the DCF counteracts the positive dispersion in the transmission fiber. Similarly, positive dispersion DCF can be used to counteract the negative dispersion of some fiber types. While DCF provides adequate levels of dispersion compensation, it is difficult to produce DCF that also simultaneously compensates the dispersion slope. As transmission lengths between regeneration points increase and data rates increase, the need to compensate dispersion slope is paramount. Uncompensated dispersion slope results in system penalty and can significantly shorten transmission distances and/or channel counts. Ideally, upon reception each channel should have the same amount of net dispersion so that the net dispersion slope is zero.
Referring to FIG. 1, an existing dispersion management technique is illustrated on a fiber link 10. Existing Dispersion management techniques rely on a high-slope dispersion compensating module (i.e., Type III DCMs) to compensate for high dispersion slope transmission fiber such as Large Effective Area Fiber (LEAF, available from Corning, Inc. of Corning, N.Y.), Truewave fibers (TW, available from OFS of Norcross, Ga.), and the like. DCFs 12 are distributed along the fiber link 10. The DCFs 12 do not provide perfect compensation across the entire C-Band, i.e. approximately 1530 nm to 1565 nm, as illustrated in a dispersion map 14 that is shown corresponding to various points on the fiber link 10. Additional compensation is achieved by using band compensating DCMs 16 to trim the dispersion at the receiver. For certain fiber types such as Dispersion Shifted Fiber (DSF) specified in ITU-T G.653 and LS fibers (available from Corning, Inc.), where the zero dispersion is within the transmission window, the dispersion compensation almost always is based on band-level compensation. Band-level compensation is typically a correction that is applied only before the receiver.
Conventional dispersion management techniques have several shortcomings. First, the residual dispersion error from a slope mismatch between the DCM and transmission fiber can only be compensated at the receiver (Rx) node. Also, the compensation at any node is static, and is usually computed by a design tool, based on where the band is added in the network. Any change in the traffic pattern requires a new band DCM. Thirdly, because the dispersion compensation is banded, this forces the path dispersion for all the channels in the band to within a narrow window. This has the effect of banding the channels, which may not be desirable for Reconfigurable Optical Add-Drop Multiplexed (ROADM)-based mesh networks. Finally, the most important limitation of band compensation is that it forces a banded de-multiplexor architecture, which is not the most cost efficient solution.
Referring to FIG. 2, a graph 20 illustrates a typical dispersion error due to slope mismatch between TW-classic (TWC) fiber plant and a type III DCM. The graph 20 is a plot of dispersion error 22 in time (ps) versus wavelength (nm). If the dispersion error 22 is minimized at the center of the band (i.e., approximately 1550 nm), the error is typically ±30 ps/nm for every 100 km of transmission fiber. The error contribution from DSF fiber plant with the zero dispersion at the center of the transmission band is much higher at approximately ±120 ps/nm for every 100 km of transmission fiber
Thus there exists a need to provide dispersion slope and dispersion map management without the aforementioned limitations.